Underwater acoustic direction sensing techniques are used, for example, to accurately determine the position of a sensor towed behind a ship. Such sensors are conventionally used in underwater acoustic surveying and positioning applications. However, the accuracy of measurements made by such sensors is governed by the accuracy with which the position of the sensor itself can be determined. Suppose for example that a sensor towed behind a ship is to produce a measurement representative of the position of a point on the seabed relative to the sensor. If the position of the sensor, relative to the ship, at the time the measurement is made can be determined only approximately, then the location of the point on the seabed relative to the ship can also be determined only approximately, regardless of the precision of the measurement made by the sensor.
The prior art has evolved a variety of underwater acoustic direction sensing techniques. A technique known as "long baseline" sensing is used to measure the time required for an underwater transmitted pulse to reach each of several widely separated underwater receivers, following which conventional triangulation methods are used to determine the position of the underwater transmitter relative to the receivers. A technique known as "short baseline" sensing is used in situations where it is impractical to locate several sensors along an extended receiver baseline. For example, the receivers may be aboard a ship and so the receiver spacing (i.e. the "baseline") is limited by the physical dimensions of the ship. Triangulation is again used to determine the position of a remote transmitter relative to the receivers, although the shortened receiver baseline impairs the accuracy with which the transmitter position can be determined; the essential difficulty being that if the distance between the receivers exceeds half the wavelength of the transmitted signal then one cannot unambiguously resolve the phase angle of the transmitted signal. It happens that the phase angle of the received signal, together with the received signal amplitude information, permits more accurate time (i.e. signal propagation delay; and therefore distance) measurements to be made than those attainable by working with only the amplitude information contained in a received signal. Thus, the problem is to accurately and unambiguously measure the phase angle of the received signal relative to that of the transmitted signal. In a technique known as "ultra-short baseline" sensing the receivers are spaced apart by a distance equal to one half the wavelength of the transmitted signal, thereby facilitating unambiguous resolution of the phase angle of the received signal relative to that of the transmitted signal. However, the accuracy of a bearing measurement based on phase angles detected at two receivers increases as the distance between the receivers increases. Accordingly, more accurate measurements can be made if the receivers are separated by many wavelengths, provided the aforementioned phase ambiguity can be resolved.
Further factors affect the accuracy with which the bearing of a signal transmitter may be determined relative to a remote signal receiver. For example, the water through which the signal is transmitted normally moves with a component of velocity perpendicular to the acoustic path between the transmitter and receiver, thereby causing the bearing of the transmitter relative to the receiver to appear to deviate from the actual bearing by a factor which is governed by the aforementioned component of velocity and by the propagation velocity of the transmitted signal. Moreover, fluctuations in water temperature establish a sound speed gradient between the transmitter and receiver which causes further apparent deviation of the transmitter bearing relative to the receiver. Yet another factor which conventionally degrades the accuracy with which the bearing of the transmitter relative to the receiver may be determined is that the direct signal paths between the transmitter and the separated receivers are affected by sound speed differences between the two paths caused by variations in the physical characteristics of the water which the signals pass through in traversing each path.
The inventors have developed a method of determining the bearing of a signal transmitter relative to a remote signal receiver consisting of two or more receivers which are separated by many wavelengths. The method resolves the aforementioned phase ambiguity problem and also provides for correction of the errors introduced by the aforementioned factors, thereby yielding highly accurate position measurements.